The present invention relates to locks, and, more particularly, to door locks of a type which can be readily modified to be operated by a variety of keys.
A conventional door lock utilizes a pair of escutcheons mounted on opposite sides of the door and connected by a face plate that covers the door edge. A bolt is movable laterally between the escutcheons so that it can be projected through the face plate and into a socket in a stationary door frame. Movement of the bolt is controlled by a lock cylinder to which it is connected by a bolt linkage.
The cylinder is the heart of the lock and typically contains an eccentrically located core in which a key can be inserted. A key of the correct configuration interacts with tumblers or other mechanisms to permit the core to be rotated, thereby causing the desired bolt movement. Alternatively, the cylinder can be actuated by a magnetically encoded key or by a thumb turn.
The lock cylinder is usually threaded externally and screwed into an aperture in one of the escutcheons. In a conventional mortise-type lock, a set screw extends inwardly from the edge of the door to engage a groove on the cylinder, preventing the cylinder from being rotated. To remove the cylinder, it is necessary to have access to the set screw which is covered by the frame when the door is closed, so that only a person who has a key to open the door can disassemble the lock.
The lock construction described above often makes it difficult to remove a cylinder, particularly if it has been in place for a long time and became bound tightly to the escutcheon. It has also been found that, in the case of an unusually wide door, the cylinder may not be long enough for the set screws to engage the cylinder groove.
A lock of conventional construction is particularly disadvantageous in situations in which it is desirable to change lock cylinders frequently and the process of disengaging the set screw and then unscrewing the cylinder is time-consuming and inconvenient. It should be remembered that lock cylinders have close threads that require many turns and care must be taken to avoid damaging the thread or defacing the lock while gripping and rotating the cylinder.
Situations in which it is desirable to change the lock cylinders frequently may arise from the need to more strictly control access to certain areas of a building during specified time periods. For example, personnel may be provided with keys that admit them to certain rooms during regular working hours, but these persons must be prevented from entering those rooms during non-working hours. A common practice is to alter the locks at the beginning and end of a work shift so that these keys will be usable only during the intended hours. It will be apparent that in this situation the rather tedious conventional method of changing lock cylinders is unsatisfactory.
One known solution to the above problem is to utilize a lock cylinder having a removable core. This core is adapted to receive two different keys. The first key merely opens the lock, while the second is a "control key" that allows the core to be removed from the cylinder. It is possible to control access by using the control key to change the core, leaving the rest of the lock unaffected. A disadvantage of this system, however, is that a removable core is complex and expensive. Moreover, a large portion of the core must be dedicated to control key operation, with the result that the portion of the cylinder available for operation by other keys is of reduced size and the lock is proportionately less secure. Another disadvantage of removable core cylinders is that they do not facilitate replacement of the entire cylinder since only the core is readily removable. It is often desired to replace the cylinder to substitute one that has a different keyway configuration, one that uses a magnetically encoded key, or one that incorporates a thumb turn.
It is a primary objective of the present invention to provide an improved door lock in which the entire cylinder can be installed and removed easily and quickly, without sacrificing security, thereby overcoming disadvantages of the previously known interchangeable core locks.